gnupic: Thread: PIC16F877 & USART

Hi. I'm trying to get a PIC16F877 to work at 9600 baud. It works fine at
2400, with BRGH=0, but I can't seem to get reliable operation at 9600 with
BRGH=1.

I thought I read somewhere that one shouldn't even try to use BRGH=1, but
all of the examples I've seen on the Microchip website and Piclist.com seem
to do it.

FWIW, it seemed to work better when the SPRG value was set to 26 instead of
25, the value everything says I should use.

I have a MAX232 from TI between the PIC and the PC. The handshaking lines
are all hard-wired.

I try to send ascii '0' through '9' from my terminal program, and display
those values on 8 LEDs. At 2400 baud, it works great. At 9600, I get
incorrect (although consistently) values.

Any ideas? Thanks!

-- 
Rick

Take a look at this code at
http://www.ee.ualberta.ca/~ee401/debug/debug877.asm  Actually, here is
the snipplet of code ...

; receives: nothing
; uses: W
; returns: nothing
;*******************************************************************
initializeUART		; Set up for hardware flow control

	banksel	BANK0		; Set to bank0
	bcf	PORTC,Tx_PIN	; Initial condition for the Tx line
	bcf	PORTC,Rx_PIN	; Initial condition for the Rx line
	bcf	PORTB,RTS_PIN	; Initial condition for the RTS line
	bsf	PORTB,CTS_PIN	; Initial condition for the CTS line

	banksel	BANK1		; Set to bank1
	bsf	TRISB,RTS_PIN	;  set RTS Pin to input 
	bcf	TRISB,CTS_PIN	;  set CTS Pin to output 
	bsf	TRISC,Tx_PIN	;  set Rx and Tx Pins to default as 
	bsf	TRISC,Rx_PIN	;  high impedance
	 
	bcf	PIE1,RCIE	; Disable uart interrupts
	bcf	PIE1,TXIE

				; Set baud Rate to one of the following
;	movlw	0xCF		; 300 baud 
;	movlw	0x67		; 2400 baud 
	movlw	0x19		; 9600 baud (see table in pic16f873.pdf)
;	movlw	0xC		; 19200 baud 
	movwf	SPBRG

	movlw	0x24		; Initialize Tx status and control register
	movwf	TXSTA

	banksel	BANK0		; Set to bank0
	movlw	0x90		; Initialize Rx status and control register
	movwf	RCSTA

	return

Or take a closer look at what the University of Alberta - Electrical and
Computer Engineering department is doing with the PIC16F873 and
PIC16F877. (http://www.ee.ualberta.ca/~ee041)  

Please keep in mind that the PIC code written on this site was done by
students for students.   

Hopefully this helps.


Rick Mann wrote:
> 
> Hi. I'm trying to get a PIC16F877 to work at 9600 baud. It works fine at
> 2400, with BRGH=0, but I can't seem to get reliable operation at 9600 with
> BRGH=1.
> 
> I thought I read somewhere that one shouldn't even try to use BRGH=1, but
> all of the examples I've seen on the Microchip website and Piclist.com seem
> to do it.
> 
> FWIW, it seemed to work better when the SPRG value was set to 26 instead of
> 25, the value everything says I should use.
> 
> I have a MAX232 from TI between the PIC and the PC. The handshaking lines
> are all hard-wired.
> 
> I try to send ascii '0' through '9' from my terminal program, and display
> those values on 8 LEDs. At 2400 baud, it works great. At 9600, I get
> incorrect (although consistently) values.
> 
> Any ideas? Thanks!
> 
> --
> Rick
> 
> ---------------------------------------------------------------------
> To unsubscribe, e-mail: ####@####.####
> For additional commands, e-mail: ####@####.####

What is your clock rate for the pic? There are multiple tables for different
clock rates in the documentation.

Here are my settings for a 20Mhz pic and 9600baud:

  SPBRG = 129;          // 9600bps
  BRGH = 1;						// High speed baud rate generation on
  RCSTA = 0;
  SYNC = 0;						// Async
  TX9  = 0;						// 8bits TX please
  RX9  = 0;						// 8bits RX please
  SPEN = 1;						// Enable serial port
  CREN = 1;						// Enable receiver
  TXEN = 1;						// Enable transmitter
  TXIE = 0;						// Disable transmitter interrupts


I hope this helps...

Andreas, N6NU

-----Original Message-----
From: Rick Mann ####@####.####
Sent: Saturday, December 28, 2002 1:14 PM
To: PICList; GNU PIC List
Subject: PIC16F877 & USART


Hi. I'm trying to get a PIC16F877 to work at 9600 baud. It works fine at
2400, with BRGH=0, but I can't seem to get reliable operation at 9600 with
BRGH=1.

I thought I read somewhere that one shouldn't even try to use BRGH=1, but
all of the examples I've seen on the Microchip website and Piclist.com seem
to do it.

FWIW, it seemed to work better when the SPRG value was set to 26 instead of
25, the value everything says I should use.

I have a MAX232 from TI between the PIC and the PC. The handshaking lines
are all hard-wired.

I try to send ascii '0' through '9' from my terminal program, and display
those values on 8 LEDs. At 2400 baud, it works great. At 9600, I get
incorrect (although consistently) values.

Any ideas? Thanks!

--
Rick


---------------------------------------------------------------------
To unsubscribe, e-mail: ####@####.####
For additional commands, e-mail: ####@####.####

on 12/31/02 1:27 PM, Andreas Junge - DSLX at ####@####.#### wrote:

> What is your clock rate for the pic? There are multiple tables for different
> clock rates in the documentation.

Oh, I forgot to mention that, sorry. 4.000 MHz.

> Here are my settings for a 20Mhz pic and 9600baud:
> 
> SPBRG = 129;          // 9600bps
> BRGH = 1;                        // High speed baud rate generation on
> RCSTA = 0;
> SYNC = 0;                        // Async
> TX9  = 0;                        // 8bits TX please
> RX9  = 0;                        // 8bits RX please
> SPEN = 1;                        // Enable serial port
> CREN = 1;                        // Enable receiver
> TXEN = 1;                        // Enable transmitter
> TXIE = 0;                        // Disable transmitter interrupts

So, here's the thing. I appreciate everyone giving me info about how to set
up the values, but the problem I'm facing is that I *did* set up the values
correctly, using both formulas (the one from Microchip and the one that
accounts for rounding errors).

The formulas work for 2400 baud, but don't seem to work for 9600 baud.

I finally shipped a couple of units at 2400 baud. It's suitable for my
application (simple control protocol), but I've run into more problems.

Basically, someone sends something like "I3" to my device. My device then
responds with "I3". It does this by executing synchronous code to send one
byte, then the next.

After a time, it will stop sending the second byte. However, if you command
it again, it will send the first byte. I'll probably have to post the code
at some point, but that's the new problem (still haven't solved the old
ones).

Happy New Year to all!


-- 
Rick

At 13:14 28.12.02 -0800, you wrote:
>Hi. I'm trying to get a PIC16F877 to work at 9600 baud. It works fine at
>2400, with BRGH=0, but I can't seem to get reliable operation at 9600 with
>BRGH=1.

If possible check your signals with scope, maybe the problem lies somewhere else.

>I thought I read somewhere that one shouldn't even try to use BRGH=1, but
>all of the examples I've seen on the Microchip website and Piclist.com seem
>to do it.

This BRGH=1 bug applied to PIC16C77 (or 74?), not to the 16F877.

>I try to send ascii '0' through '9' from my terminal program, and display
>those values on 8 LEDs. At 2400 baud, it works great. At 9600, I get
>incorrect (although consistently) values.

Check your hardware this way - remove the PIC from its socket, connect the RC6 and RC7 contacts with a piece of wire. Now your terminal program should echo all the characters typed. If this works O.K., try following main loop, which should echo all the characters as well:

;setup serial port
....
     bcf _GIE ;(or don't set it)
main:btfss _RCIF
     goto main
     movf rcreg,w
     movwf txreg
     goto main

Regards,

Josef

>Basically, someone sends something like "I3" to my device. My device then
>responds with "I3". It does this by executing synchronous code to send one
>byte, then the next.

Just curious - what do you call 'synchronous code'?

Josef

on 1/1/03 11:24 PM, Snail Instruments at ####@####.#### wrote:

> Just curious - what do you call 'synchronous code'?

I guess you could say it polls. The code loads TXREG, then sits in a loop
waiting for the txmt flag to be set, then loads TXREG with the second byte
to send.

By contrast, async code would load TXREG, then go off an do something else,
waiting for the txmt interrupt to occur.

-- 
Rick

On Wed, 1 Jan 2003, Rick Mann wrote:

>on 12/31/02 1:27 PM, Andreas Junge - DSLX at ####@####.#### wrote:
>
>> What is your clock rate for the pic? There are multiple tables for different
>> clock rates in the documentation.
>
>Oh, I forgot to mention that, sorry. 4.000 MHz.
>
>So, here's the thing. I appreciate everyone giving me info about how to set
>up the values, but the problem I'm facing is that I *did* set up the values
>correctly, using both formulas (the one from Microchip and the one that
>accounts for rounding errors).



-- 
Jeff Jackowski
        http://ro.com/~jeffj/

On Wed, 1 Jan 2003, Rick Mann wrote:

>on 12/31/02 1:27 PM, Andreas Junge - DSLX at ####@####.#### wrote:
>
>> What is your clock rate for the pic? There are multiple tables for different
>> clock rates in the documentation.
>
>Oh, I forgot to mention that, sorry. 4.000 MHz.
>
>So, here's the thing. I appreciate everyone giving me info about how to set
>up the values, but the problem I'm facing is that I *did* set up the values
>correctly, using both formulas (the one from Microchip and the one that
>accounts for rounding errors).
>
>The formulas work for 2400 baud, but don't seem to work for 9600 baud.

Bother, I pressed the wrong button. Sorry for the extra post.

I wrote some code in C to do exactly what you are trying to do, and it 
works fine with the published values. It isn't for a free compiler, but it 
does mess with the hardware registers directly. If you think it might 
help, you can get the code at 
http://home.hiwaay.net/~jeffj1/projects/index.html from the top link. The 
relavent stuff is in uart.c.

The only major difference I could find between what your code does and 
what mine does is that I set the USART enable bit after setting all other 
values. I'm not sure if that is needed, but it seemed like a good idea at 
the time.

-- 
Jeff Jackowski
        http://ro.com/~jeffj/

On Saturday 04 January 2003 19:48, somebody wrote
> On Wed, 1 Jan 2003, Rick Mann wrote:
> >on 12/31/02 1:27 PM, Andreas Junge - DSLX at ####@####.#### 
wrote:
> >> What is your clock rate for the pic? There are multiple tables for
> >> different clock rates in the documentation.
> >
> >Oh, I forgot to mention that, sorry. 4.000 MHz.
> >
> >So, here's the thing. I appreciate everyone giving me info about how
> > to set up the values, but the problem I'm facing is that I *did*
> > set up the values correctly, using both formulas (the one from
> > Microchip and the one that accounts for rounding errors).
> >
> >The formulas work for 2400 baud, but don't seem to work for 9600
> > baud.

Can you write a test program using your code to output a continual 
stream of 0xCC (=1010 1010) and 'scope the frequency? A little bit of 
copy'n'paste would at least quantify your error (too FAST, or too SLOW).
You need to divide your problem into fragments, and fix the one that's 
broken.

If you are seriously stuck, Microchip do have help lines. I got the 
phone number for the English one.
-- 
	Regards,

	Declan Moriarty